An important factor in the use of lasers is the precision with which the beam is controlled. Various sensors are used to measure the beam location, usually by measuring variations in beam intensity from a null state to provide outputs to a control system to adjust the beam alignment.
The prior art includes a variety of different laser control methods radiometers (Clifford U.S. Pat. No. 3,348,047), optics (Kunitsugo U.S. Pat. No. 4,867,560), radar (Lloyd U.S. Pat. No. 4,738,531), florescence (McClung, Jr. U.S. Pat. No. 4,700,068), and mirrors (Anderson U.S. Pat. No. 3,579,140). The sensor of choice is usually some type of photoconductive device. U.S. Pat. No. 3,723,013 discloses four quadrant silicon photodetectors used to control the positioning of a laser beam. U.S. Pat. No. 4,243,888 teaches the use of a silicon disk having thermoelectric properties.
Any material which permits optical radiation to pass through it absorbs a certain percentage of the incident power and dissipates it in the form of heat. This raises the temperature of the illuminated region and causes a temperature rise in the material which can be detected and measured by appropriately placed thermal sensors. In any sensor system, based on thermoconductive sensors, it is desirable to be able to accurately sense changes in thermoconductivity while minimizing the heat absorbed by the sensor.
The present invention embodies a sensor which has superior heat dissipation and enhanced detection characteristics when compared to prior art sensors.
Broadly the invention comprises a thermoconductive sensor, a dielectric layer to provide electrical insulation between the sensor and a supporting substrate, a heat sensitive material, and a layer to reflect heat embedded within the material including a plurality of extending fingers to conduct heat absorbed by the material. The dielectric material may include such materials as SiO.sub.2, Si.sub.3 N.sub.4, Si.sub.2 ON.sub.2, Al.sub.2 O.sub.3. The layer embedded within the material may include platinum or platinum rhodium. Preferably, the tips of the fingers extend through the material and into heat exchange relationship with the substrate which also functions as a heat sink. A plurality of serially connected thermocouples embedded within the material and spaced apart from the fingers provide an output corresponding to changes in thermoconductivity of the material. In the preferred embodiment, the reflective material is refractive. This feature, in combination with the heat sinking, aids in minimizing the impact on the sensor if the main beam strikes the sensor. Further, the sensor surface exposed to the beam is orthogonal with reference to the longitudinal axis of the beam. This also minimizes the incident power on the sensor surface and thus lessens the likelihood of damage.